Baker's Cyst

A Baker's cyst is a swelling behind the knee that may develop . It is generally found that the lubricating fluid inside the knee joint which is filled with fluid . There is an underlying problem with your knee such as osteoarthritis , if it occurs the most . Symptoms of pain behind the knee , swelling and heaviness can include . Rarely , a Baker 's Cyst ( split open ) cleavage and a deep vein thrombosis can cause similar symptoms . A Baker's cyst often gets better by itself , and time disappears. However , if you have symptoms associated with the different treatments that can help .

Some knee joint anatomy

The first diagram below illustrates a typical normal knee joint looking from the side.

The joint capsule is a thick structure that surrounds your whole knee and gives it some support. It is lined by a special membrane called the synovium. The synovium produces a fluid called synovial fluid. This fluid acts as a lubricant within your knee joint and helps to cushion it during movement.

There are also various tissue pouches called bursae next to the knee. A bursa is a small sac of synovial fluid with a thin lining. Bursae are normally found around joints and in places where ligaments and tendons pass over bones. They help to reduce friction and allow maximal range of motion around joints. The bursa at the back of your knee is called the popliteal bursa.

Each knee joint also contains a medial and lateral meniscus. These are thick rubber-like pads of cartilage tissue. The menisci cartilage sit on top of, and are in addition to, the usual thin layer of cartilage which covers the top of the tibia (one of the bones of the lower leg). They act as shock absorbers to absorb the impact of the upper leg on the lower leg and also help to improve smooth movement and stability of the knee.

What causes a Baker's cyst?

There are two ways in which a Baker's cyst may form:

What is a Baker's cyst?

A Baker's cyst is a fluid-filled swelling that can develop behind the knee. It is named after a doctor called William Baker who first described this condition in 1877. It is also sometimes called a popliteal cyst, as the medical term for the area behind the knee is the popliteal fossa.

The cyst can vary in size from a very small cyst to a large cyst that is a number of centimetres across. Rarely, a Baker's cyst can develop behind both knees at the same time.

A primary Baker's cyst

A Baker's cyst may develop just behind an otherwise healthy knee joint. This type of cyst is sometimes referred to as a primary or idiopathic Baker's cyst. It usually develops in younger people and children.

It is thought that in this type of Baker's cyst there is a connection between the knee joint and the popliteal bursa behind the knee. This means that synovial fluid from inside the joint can pass into the popliteal bursa and a Baker's cyst can form.

A secondary Baker's cyst

Sometimes a Baker's cyst can develop if there is an underlying problem within the knee, such as arthritis (including osteoarthritis and rheumatoid arthritis), or a tear in the meniscal cartilage that lines the inside of the knee joint. This type of Baker's cyst is the most common. It is sometimes referred to as a secondary Baker's cyst.

In a secondary Baker's cyst, the underlying problem within the knee joint causes too much synovial fluid to be produced within the joint. As a result of this, the pressure inside the knee increases. This has the effect of stretching the joint capsule. The joint capsule bulges out into the back of the knee, forming the Baker's cyst that is filled with synovial fluid.